POLYPIFERA. 



55 



By the opposite extremity to that which is 

 derived from the margin of the cell, the ten- 

 tacular sheath unites with a tolerably capa- 

 cious tube, the walls of which are exceedingly 

 soft and delicate, and near the point of their 

 union we may perceive a fasciculus of fibres 

 running downwards to be inserted upon the 

 lateral walls of the cell. These fibres appear 

 to be striated transversely, and are evidently 

 muscular ; their use cannot be doubted. When 

 the animal wishes to expand itself, the mem- 

 branous sheath above referred to becomes 

 rolled outward, everting itself like the finger 

 of a glove as the tentacles advance. The 

 muscular fasciculi are thus placed between 

 the everted sheath and the alimentary canal, 

 and by their contraction they must necessarily 

 retract the whole within the cell. 



The first portion of the alimentary canal (Jig. 

 58,6) is inflated, and much wider than the rest ; 

 it forms a kind of chamber, in which the water 

 set in motion by the cilia of the tentacles ap- 

 pears to circulate freely. The walls of this 

 chamber are exceedingly delicate ; the soft 

 membrane forming them is puckered, and ap- 

 pears traversed by many longitudinal canals 

 united by minute transverse vessels ; this 

 appearance, however, may be deceptive. 



Beneath the first enlargement, the digestive 

 apparatus becomes narrower, but immediately 

 expands again, and offers at this point a cer- 

 tain number of filiform appendages (c), which 

 appear to be free and floating in the interior of 

 the cell. To the second cavity succeeds a nar- 

 row canal opening into a third dilatation, gene- 

 rally of a spherical form (d). From the last- 

 named viscus issues a kind of intestine, which 

 soon bends upon itself and becomes attached 

 to an organ of a soft and membranous tex- 

 ture, having the appearance of a caecum, and 

 which seems to be continuous superiorly with 

 the digestive tube. The latter continues its 

 progress towards the upper part of the cell, 

 and ultimately terminates by a distinct anal 

 aperture upon the upper aspect of the ten- 

 tacular sheath. The operculurn which closes 

 the cell in Flustrae and Escharae is moved by 

 two muscular fasciculi inserted into the in- 

 ternal face of this valve by the intermedium 

 of two filaments analogous to tendons ; by 

 their inferior extremity these muscles are 

 attached to the walls of the cell, and when, 

 by its own elasticity, the operculum is turned 

 back, and the mouth of the cell thus opened, 

 they by their contraction can close it like a 

 door. 



Reproduction. The first mode of repro- 

 duction observed in the Ciliobrachiate polypes 

 is by a process of gemmation from the com- 

 mon stock or creeping stem upon which the 

 animals grow. This is easily witnessed, as 

 the gemmae are met with in every progressive 

 stage of development upon the same specimen, 

 as represented mjig. 65. 



The smallest gemmae are described by Dr. 

 Farre as homogeneous in their texture, form- 

 ing little nodules on the parent stem. Those 

 further advanced were seen to present some- 

 thing like a boundary line, indicating the 



thickness of the parietes of the future cell. 

 Within this, in others, was a dark mass, 

 which in larger ones presented a rough outline 

 of the form of the complete animal. Those 

 about half grown had all the parts distinctly 

 traced out ; the retractor muscles completely 

 formed ; the tentacles short and clumsy ; the 

 walls of the alimentary canal thick, and its 

 boundaries clearly defined. 



This mode of propagation has been still more 

 completely studied by Professor Van Beneden, 

 whose opportunities of observation enabled 

 him to prosecute the inquiry more closely. 



In Pedicellina the phenomena attending the 

 gemmiparous mode of reproduction are de- 

 scribed by Professor Van Beneden as present- 

 ing the following phases of development. First, 

 there sprouts from the common stem of the 

 Bryozoon, without any determinate situation, 

 a tubercle which is but a prolongation from 

 the stem itself (fig. 65, , 8) ; this tubercle 

 extends outwards, becomes more prominent, 

 and soon swells out into a vesicle (b, 8), which 

 is" the first appearance of the new individual. 

 Up to this period the interior of this vesicle, 

 is like that of the stem itself, of which it is 

 only an extension ; but now a cellule becomes 

 visible in its centre, which forms the point 

 of departure whence the development of the 

 embryo proceeds. 



Around this primitive cell a series of other 

 very small cellules soon group themselves, 

 which seem to constitute the parietes of the 

 primitive vesicle or the blastoderm, the original 

 cell representing the vitelline cavity. The 

 bud enlarges, and as its growth proceeds the 

 internal tissue becomes thickened, so as to 

 fill it ; subsequently an indentation is appa- 

 rent on each side of the little cavity which 

 separates it into two halves, the inferior of 

 which will form the stomach, properly so 

 called, while the upper division will become 

 the anterior space between the tentacula. 



The mode of reproduction by gemmae has 

 been carefully studied in the genus Laguncula 

 (Lagenella of Farre) by the same investigator. 

 The reproductive buds sprout from the creep- 

 ing stems (fig. 61, y) which connect the indi- 

 vidual animals, appearing at first as a slight 

 prominence, that soon expands into a rounded 

 tubercle, which is the commencement of a 

 new cell. 



On close inspection, this bud is found to 

 consist of a transparent envelope, which is, 

 in fact, a continuation of the general invest- 

 ment of the polype. This rudimentary cell is 

 lined throughout with a soft membrane, having 

 its inner surface studded with minute globules, 

 by the accumulation of which the polype is 

 ultimately formed. The bud itself is hollow, 

 and communicates with the parent stem. It 

 therefore has nothing in its composition re- 

 sembling that of an egg ; neither distinct vesi- 

 cle nor vitellus ; this condition of the gemma 

 is represented in Jig. 62. 1. The new 

 formed cell soon grows taller, and its lining 

 membrane becomes thicker, and indicates the 

 commencement of the intestinal canal, which 

 is at first a simple cavity, bounded by the 



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